The novel's narrator is a widowed 58-year-old Swiss-born physician, Howard J. Rageet, who lives in New York City. His son is a pediatrician, his daughter a medical student. Rageet himself is terminally ill. He is writing a "little biography," of Mary Mallon, the infamous "healthy carrier" also known as Typhoid Mary. Rageet's grandfather, also a doctor, had kept a journal about Mary and his rivalry with his friend, (the real) George A. Soper, whose life's work became tracking Mary and proving that she was responsible for the typhoid outbreaks. Elaborating on the journal, Rageet recounts Mary's life in America.

Born Maria Anna Caduff in the same part of Switzerland as Rageet's ancestors, she arrives in New York Harbor in 1868, aged 13, on a crowded immigrant ship, a fifth of whose passengers had died en route from Europe. The dead include Mary's family. She had been taken care of by the ship's cook, who evidently both taught her to cook and used her for sex. When the ship docks, Mary tries to jump overboard, but is stopped by a physician, Dorfheimer, who smuggles her through Ellis Island and takes her home with him. He is also a pedophile, and he has sex with her. Rageet calls this kidnapping a "humane, benevolent crime." Not long after, Dorfheimer dies of typhoid fever.

Rageet's "ballad" then traces Mary's various positions as a cook (and, often, sexual object), most of which end quickly when the household is infected. She has two relationships that do not lead to the disease. One is with a small girl who has Down Syndrome. Once her connection to typhoid is suspected, the child's family hire Mary to live alone with the child and care for her, hoping the child will be infected and die. The child never becomes ill. The other is with a disillusioned anarchist, Chris Cramer. She lives with him and falls in love with him, but he is not sexually interested in her.

Soper encounters Mary when he is asked by a wealthy Oyster Bay family, her former employers, to investigate a typhoid outbreak in their household. He manages to track her down and eventually, after much resistance, she is arrested, tested, and quarantined. She escapes and continues to work as a cook until her re-arrest. Promising to try and imagine Mary's motives, Rageet breaks off his narrative. He is dying. The novel ends with a postscript written by Rageet's daughter. Implying that her father committed suicide, she tells of Mary's stroke and the last years of her life as a paraplegic, and she provides a final document, the menu for one of the very elaborate meals Mary would have cooked.